Methods of making multi-state non-volatile memory cells

ABSTRACT

A semiconductor device includes a region in a semiconductor substrate having a top surface with a first charge storage layer on the top surface. A first conductive line is on the first charge storage layer. A second charge storage layer is on the top surface. A second conductive line is on the second charge storage layer. A third charge storage layer is on the top surface. A third conductive line is on the third charge storage layer. A fourth charge storage layer has a first side adjoining a first sidewall of the first conductive line and a second side adjoining a first sidewall of the second conductive line. A fifth charge storage layer has a first side adjoining a second sidewall of the second conductive line and a second side adjoining a first sidewall of the third conductive line. Source and drain regions are formed in the substrate on either side of the semiconductor device.

BACKGROUND

1. Field

This disclosure relates generally to an integrated circuit memory, andmore specifically, to a multi-state non-volatile memory cell integrationand method of operation.

2. Related Art

One type of non-volatile memory uses traps in an insulating layer forcharge storage. One material used in such a manner is silicon nitride.Typically, the nitride charge storage layer is surrounded by otherinsulating layers such as oxide forming an oxide-nitride-oxide (ONO)structure. Charge stored within the nitride is used to manipulate athreshold voltage of the transistor, and in this manner store data.Another type of non-volatile memory uses nanocrystals for chargestorage. A conventional non-volatile memory gate cell typically existsin one of two states representing either a logical zero or a logicalone. To increase the capacity of a memory device without significantlyincreasing the size of the memory, a multi-bit memory cell may be usedthat is capable of storing more than two states. Non-volatile memorycells of this type, referred to herein as multi-state memory cells, havebeen historically implemented by controlling the amount of charge thatis injected into portions of the nitride charge storage layer.

Multi-state memory cells having nitride or nanocrystals for chargestorage and that rely on localization of charge are relatively robustbecause charge migration is minimal. More specifically, the charge doesnot spread out through the nitride layer, causing the stored logicstates to change. In multi-state non-volatile memory cells that usemultiple independent floating gates, it has been necessary to usemultiple non-self-aligned masking steps to fabricate the multiplefloating gates, significantly increasing the cost of the device due tothe increased process complexity and larger size of the memory cell.

Therefore, there is a need for a multi-state non-volatile memory devicehaving good data retention capabilities while also being inexpensive tomanufacture.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and is notlimited by the accompanying figures, in which like references indicatesimilar elements. Elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale.

FIG. 1 through FIG. 10 illustrate cross-sectional views of a multi-statenon-volatile memory cell and a method for making the multi-statenon-volatile memory cell in accordance with an embodiment.

FIG. 11 illustrates, in partial schematic diagram form, a multi-statenon-volatile memory cell and a method for programming the non-volatilememory cell in accordance with an embodiment.

FIG. 12 illustrates a top down view of a non-volatile memory cell inaccordance with an embodiment.

FIG. 13 illustrates a cross-sectional view of a portion of thenon-volatile memory cell of FIG. 12 along the line 13-13.

FIG. 14 illustrates, in partial schematic diagram form, a multi-statenon-volatile memory cell in accordance with an embodiment.

FIG. 15-FIG. 25 illustrate cross-sectional views of a multi-statenon-volatile memory cell and a method for making the non-volatile memorycell in accordance with an embodiment.

DETAILED DESCRIPTION

Generally, there is provided, a thin-film storage (TFS) multi-statenon-volatile memory cell having multiple gates spaced relatively closetogether so that inversion layers in the channel regions overlap eachother when adjacent gates are biased in a conductive state. There are nosource/drain regions between the gates. Charge storage may be bynanocrystal or SONOS (semiconductor oxide nitride oxide semiconductor).There can be any number of gates, and in one embodiment, the number ofstored states is equal to the number of gates plus one.

The semiconductor substrate described herein can be any semiconductormaterial or combination of materials, such as gallium arsenide, silicongermanium, silicon-on-insulator (SOI), silicon, monocrystalline silicon,the like, and combinations of the above.

As used herein the term metal-oxide-semiconductor and the abbreviationMOS are to be interpreted broadly, in particular, it should beunderstood that they are not limited merely to structures that use“metal” and “oxide” but may employ any type of conductor including“metal” and any type of dielectric including “oxide”. The term fieldeffect transistor is abbreviated as “FET”.

FIG. 1 through FIG. 10 illustrate cross-sectional views of a multi-statenon-volatile memory cell and a method for making the multi-statenon-volatile memory cell in accordance with an embodiment. FIG. 1illustrates a cross-section of multi-state non-volatile memory cell 10after charge storage layer 16 and conductive layer 17 are formed onsemiconductor substrate 12. Semiconductor substrate 12 can be anysemiconductor material or combination of materials, such as galliumarsenide, silicon germanium, silicon-on-insulator (SOI), silicon,monocrystalline silicon, the like, and combinations of the above.Shallow trench isolation (STI) 14 is first formed in substrate 12 inaccordance with a conventional isolation technique. In one embodiment,to form charge storage layer 16, a dielectric stack is formed oversubstrate 12 and includes a first insulating layer, a charge storagelayer, and a second insulating layer. The first insulating layer may begrown from substrate 12. In one embodiment, charge may be stored using aplurality of discrete charge storage elements such as nanocrystals. Inthe illustrated embodiment, nanocrystals are represented by the smallcircles in charge storage layer 16. These nanocrystals are typicallyformed of silicon, but the discrete storage elements may also be formedof clusters of material consisting of, for example, germanium, siliconcarbide, any number of metals, or any combination of these. In anotherembodiment, the charge storage layer may be nitride or polysilicon. Thesecond insulating layer is deposited on the charge storage layer. In oneembodiment, the second insulating layer is a deposited oxide layer.Conductive layer 17 is formed over charge storage layer 16. Conductivelayer 17 can be any type of metal, such as aluminum or copper, oranother type of conductive material such as polysilicon.

FIG. 2 illustrates a cross-section of multi-state non-volatile memorycell 10 after a photo resist layer (not shown) is formed over metallayer 17 and patterned. Conductive layer 17 and charge storage layer 16are then etched to form gates 18, 20, and 22 as lines in conductivelayer 17. The photoresist layer is then removed.

FIG. 3 illustrates a cross-section of multi-state non-volatile memorycell 10 after an insulating layer 24 is formed over gates 18, 20, 22,and over exposed portions of substrate 12. In one embodiment, insulatinglayer 24 is deposited silicon dioxide.

FIG. 4 illustrates a cross-section of multi-state non-volatile memorycell 10 after a conventional chemical mechanical polishing (CMP)procedure is used to remove a top portion of insulating layer 24. Asillustrated in FIG. 4, in one embodiment, insulating layer 24 ispolished down to the tops of gates 18, 20, and 22.

FIG. 5 illustrates a cross-section of multi-state non-volatile memorycell 10 after a photoresist layer 26 is formed over insulating layer 24and patterned to open an area between the outermost gates 18 and 22.

FIG. 6 illustrates a cross-section of multi-state non-volatile memorycell 10 after portions of insulating layer 24 are removed from betweengates 18 and 20 and from between gates 20 and 22.

FIG. 7 illustrates a cross-section of multi-state non-volatile memorycell 10 after a conductive layer 30 is deposited over a charge storagelayer 28. Charge storage layer 28 is over gates 18, 20, and 22, oversubstrate 12 between the gates, and over insulating layer 24. Chargestorage layer 28 is formed to be substantially the same as chargestorage layer 16. Conductive layer 30 may be formed from a metal orother conductive material such as polysilicon. Generally, in oneembodiment, conductive layer 30 comprises the same type of material asin gates 18, 20, and 22.

FIG. 8 illustrates a cross-section of multi-state non-volatile memorycell 10 after a conventional CMP process is used to remove a portion ofconductive layer 30. In one embodiment, conductive layer 30 and chargestorage layer 28 are removed to the tops of gates 18, 20, and 22 to formgates 19 and 21. As can be seen in FIG. 8, gates 18, 19, 20, 21, and 22are insulated from each other by only the insulating layers that form aportion of charge storage layers 16 and 28.

FIG. 9 illustrates a cross-section of multi-state non-volatile memorycell 10 after photoresist layer 32 is formed and patterned. Insulatinglayer 24 is then removed from areas not covered by patterned photoresistlayer 32 using conventional etching techniques.

FIG. 10 illustrates a cross-section of multi-state non-volatile memorycell 10 after further processing to form source region 34, drain region36, and spacers 38. Note that source region 34 and drain region 36 alsoinclude extensions under spacers 38. Also formed, but not illustrated,are other features necessary to complete a device, such as for example,silicide, contacts, various implant and clean steps, and additionalmetal layers.

FIG. 11 illustrates, in partial schematic diagram form, multi-statenon-volatile memory cell 40 and a method for programming thenon-volatile memory cell in accordance with an embodiment. Multi-statenon-volatile memory cell 40 is a schematic representation of multi-statenon-volatile memory cell 10. Multi-state non-volatile memory cell 40includes five gate terminals labeled G1-G5 and includes state storageunits 42, 44, 46, 48, and 50. A resistor symbol under each of gatesG1-G5 indicates a channel portion controlled by each of the gates.Because the cells are separated only by charge storage layer 28,inversion layers of the cell overlap. The overlapping inversion layerscause a continuous conductive channel to be formed between the source(S) and drain (D) terminals of memory cell 40 when all of gates G1-G5are biased correctly.

The voltages used to program various state storage units of multi-statenon-volatile memory 40 are illustrated in FIG. 11. As illustrated, thesource (S) and drain (D) are connected to power supply voltages VSS andVDD, respectively. In one embodiment, VSS is ground, and VDD is apositive power supply voltage. The gates will receive one of threevoltages VP, V1, or V2. Programming voltage VP is a relatively highprogramming voltage. Voltage V1 is applied to state storage units thathave not been programmed and are not being programmed in the currentprogramming cycle. Voltage V2 is applied to state storage units thathave already been programmed in a previous programming cycle. Voltage V1is a voltage level above a threshold voltage of the state storage unit(SSU) when the SSU is in the unprogrammed state. Voltage V2 is a voltagelevel above a threshold voltage of the state storage unit when the SSUis in the programmed state. For simplicity, voltage V1 can be set equalto voltage V2.

State storage units 42, 44, 46, 48, and 50 are programmed in orderstarting with SSU 50. In the example of FIG. 11, three state storageunits are programmed using three programming cycles. Programming beginswith all of the state storage units in an unprogrammed state, or erasedstate. To program SSU 50, relatively high programming voltage VP isprovided to gate G5 while voltage V1 is provided to gates G1-G4. VoltageVP is high enough to cause Fowler-Nordheim (F-N) tunneling of chargecarriers. The actual voltages depend, at least in part, on the processtechnology used to manufacture the multi-state non-volatile memory. Toprogram state storage unit 48, programming voltage VP is provided togate G4 while voltage V1 is provided to gates G1-G3 and voltage V2 isprovided to gate G5. To program state storage unit 46, programmingvoltage VP is provided to gate G3 while voltage V1 is provided to gatesG1 and G2 and voltage V2 is provided to gates G4 and G5.

Multi-state memory cell 40 is read by applying a read voltage to allfive gates G1-G5. The drain/source current is sensed to determine thestored logic state. Multi-state memory cell 40 can store up to sixdifferent logic states. Generally, in other embodiments having more orfewer gates, the multi-state memory cell can store a number of statesequal to the number of gates plus one. The number of gates is limited bythe ability of the comparator circuitry to sense and differentiate thecurrents among the various states.

Multi-state memory cell 40 is erased by applying a relatively high erasevoltage VE to any or all five gates G1-G5 with the source (S) and drain(D) connected to power supply voltage VSS. In one embodiment, VSS isground and voltage VE is a negative voltage.

FIG. 12 illustrates a top down view of a multi-state non-volatile memorycell in accordance with another embodiment. In FIG. 12, active regions13 and 15 are surrounded by isolation region 14. Gates 18-22 are formedover active regions 13 and 15 using lines of N+ polysilicon. The gatesare separated by charge storage layer 28. Source/drain regions 34 and 36are formed adjacent to gate 18 and gate 22. P− regions 52-56 are formedin the polysilicon between the N+ regions and over the isolation region14 between active regions 13 and 15. As discussed above regardingmulti-state memory cells 10 and 40, state storage units are formed bygates 18-22 and their underlying channel regions. Another memory cell isformed over active region 15 having gates 18′-22′ formed in the same N+polysilicon as gates 18-22. In FIG. 12, additional state storage unitsare formed laterally in the polysilicon layers. For example, one statestorage unit has a gate formed by P− region 52, and a correspondingchannel region in P− region 53. The gate and channel region areseparated by charge storage layer 28. Source and drain regions areformed by N+ regions 19 and 19′. Likewise, another state storage unit isformed laterally by P− region 53 (gate), charge storage layer 28, P−region 54 (channel), and N+ regions 20 (source/drain) and 20′(source/drain). As can be seen in FIG. 12, four state storage units canbe formed using the five N+ polysilicon gate conductors used to formgates 18-22 and gates 18′-22′. Cross-sectional views of the multi-statenon-volatile memory cell of FIG. 12 are illustrated along line 13-13(FIG. 13) and along line 25-25 (FIG. 25).

FIG. 13 illustrates a cross-sectional view of a portion of themulti-state non-volatile memory cell of FIG. 12 along the line 13-13.FIG. 13 shows substrate 12 and isolation region 14 formed thereon.Charge storage layers 16 and 28 and polysilicon gates 52-56 are formedas described below with reference to FIGS. 15-25. In operation, chargeis stored in the more vertically positioned portions of charge storagelayer 28 between the gates. Current flows orthogonal to the plane of thefigure. Five gates are illustrated in FIG. 13; however, in otherembodiments a different number of gates can be formed. The number ofadditional state storage units formed this way is equal to the number ofgates minus one. During a read operation, each vertical, programmed,charge storage region makes the adjacent polysilicon region harder toinvert, thereby reducing cell current. Programming and erasing can beaccomplished using F-N tunneling.

FIG. 14 illustrates a schematic representation of a multi-statenon-volatile memory cell 59 consistent with the embodiment illustratedin FIG. 12. Multi-state non-volatile memory cell 59 includes statestorage units 60-68. State storage units 60-64 are substantially thesame as state storage units 42, 44, 46, 48, and 50 of FIG. 11. Statestorage units 65-68 are formed laterally in the polysilicon layers usedto form gates 52-56.

Programming of state storage units 60-64 is the basically the same asdescribed above regarding the embodiment of FIG. 11. A method forprogramming one or more of state storage units 65-68 is describedherein. To program, for example, SSU 68, P− poly region 56 (gate) isprovided with voltage VP, N+ poly region 21 (source) is set to VSS, N+poly region 21′ (drain) is set to VSS, and P− poly region 55 (channel)is set to VSS, N+ poly regions 18-20 and 18′-20′ are set to VSS, and P−poly regions 52-54 are set to VSS. (In the illustrated embodiment, VSSis ground and VDD is a positive voltage.) VP is high enough for F-Ntunneling. The actual voltages depend, at least in part, on the processtechnology used to manufacture the multi-state non-volatile memory. TheP− poly region 55 channel contact acts as a well tie, and channelcurrent conduction is along the sidewall of P− poly region 55 closest toP− poly region 56. To avoid disturbing data stored in the horizontalstate storage units, p-type substrate 12 receives a voltage of V1, whereV1 is a voltage between VP and VSS that is not high enough for F-Ntunneling to VSS or VP. The N+ source and drain regions of thehorizontal state storage units are set to V1 to maintain low leakage.

To program state storage unit 67 after programming state storage unit68, P− poly regions 55 and 56 are set to VP (P− poly region 55 is thegate), N+ poly region 20 is set to VSS (source), N+ poly region 20′ isset to VSS (drain), and P− poly region 54 is set to VSS (channel). N+poly regions 18, 19, 18′ and 19′ are set to VSS, and P− poly regions52-53 are set to VSS.

To erase, for example, SSU 68, P− poly region 56 (gate) is set to anerase voltage (VE), and N+ poly region 21, N+ poly region 21′ and P−poly region 55 (channel) are set to VSS. N+ poly regions 18-20 and18′-20′ are set to VSS, and P− poly regions 52-54 are set to VSS. Erasevoltage VE is high enough for F-N tunneling. The actual voltage depends,at least in part, on the process technology used to manufacture themulti-state non-volatile memory. The P− poly region 55 channel contactacts as a well tie. To avoid disturbing data stored in the horizontalstate storage units, p-type substrate 12 receives a voltage of V2, whereV2 is a voltage between VE and VSS that is not high enough for F-Ntunneling to VSS or VE. The N+ source and drain regions of thehorizontal state storage units are set to V2 to maintain low leakage.

To read, for example, state-storage unit 68, P− poly region 56 (gate) isset to a low read voltage (VR), N+ poly region 21 (source) is set toVSS, N+ poly region 21′ (drain) is set to approximately ½ VDD, and P−poly region 55 (channel) is set to VSS. N+ poly regions 18-20 and18′-20′ are set to VSS, and P− poly regions 52-54 are set to VSS. TheP-poly region 55 channel contact acts as a well tie, and channel currentconduction is along the sidewall of P− poly region 55 closest to P− polyregion 56. To minimize the impact of system noise and process variation,reads can be done from both sides of the same charge storage region andthe output currents can be averaged. To avoid disturbing data stored inthe horizontal state storage units, p-type substrate 12 and the N+source and drain regions of the horizontal state storage units are setto VSS.

FIG. 15-FIG. 25 illustrate cross-sectional views of a multi-statenon-volatile memory cell and a method for making the non-volatile memorycell in accordance with an embodiment. FIG. 15 illustrates across-section of multi-state non-volatile memory cell 80 after chargestorage layer 86 and polysilicon layer 96 are formed on semiconductorsubstrate 82. Semiconductor substrate 82 can be any semiconductormaterial or combination of materials, such as gallium arsenide, silicongermanium, silicon-on-insulator (SOI), silicon, monocrystalline silicon,the like, and combinations of the above. Shallow trench isolation (STI)84 is first formed in substrate 82 in accordance with a conventionalisolation technique. In one embodiment, to form charge storage layer 86,a dielectric stack is formed over substrate 82 and includes a firstinsulating layer, a charge storage layer, and a second insulating layer.The first insulating layer may be grown from substrate 82. In oneembodiment, charge may be stored using a plurality of discrete chargestorage elements such as nanocrystals. In the illustrated embodiment,nanocrystals are represented by the small circles in charge storagelayer 86. These nanocrystals are typically formed of silicon, but thediscrete storage elements may also be formed of clusters of materialconsisting of, for example, germanium, silicon carbide, any number ofmetals, or any combination of these. In another embodiment, the chargestorage layer may include nitride or polysilicon instead ofnanocrystals. The second insulating layer is deposited on the chargestorage layer. In one embodiment, the second insulating layer is adeposited oxide layer. In one embodiment, polysilicon layer 96 is formedby depositing polysilicon over charge storage layer 86.

FIG. 16 illustrates a cross-section of multi-state non-volatile memorycell 80 after a photo resist layer (not shown) is formed overpolysilicon layer 96 and patterned. Polysilicon layer 96 and chargestorage layer 86 are then etched to form gates 90, 92, and 94 inpolysilicon layer 96. The photoresist layer is then removed.

FIG. 17 illustrates a cross-section of multi-state non-volatile memorycell 80 after an insulating layer 97 is formed over gates 90, 92, and94, and over exposed portions of substrate 82. In one embodiment,insulating layer 97 is deposited silicon dioxide.

FIG. 18 illustrates a cross-section of multi-state non-volatile memorycell 80 after a conventional chemical mechanical polishing (CMP)procedure is used to remove a top portion of insulating layer 97. Asillustrated in FIG. 18, in one embodiment, insulating layer 97 ispolished down to the tops of gates 90, 92, and 94.

FIG. 19 illustrates a cross-section of multi-state non-volatile memorycell 80 after a photoresist layer 106 is formed over insulating layer 97and patterned to open an area between outermost gates 90 and 94.

FIG. 20 illustrates a cross-section of multi-state non-volatile memorycell 80 after portions of insulating layer 97 are removed from betweengates 90 and 92 and from between gates 92 and 94.

FIG. 21 illustrates a cross-section of multi-state non-volatile memorycell 80 after a polysilicon layer 89 is deposited over a charge storagelayer 88. Charge storage layer 88 is over gates 90, 92, and 94, oversubstrate 82 between the gates, and over insulating layer 97. Chargestorage layer 88 is formed to be substantially the same as chargestorage layer 86. Polysilicon layer 89 may be formed by depositingpolysilicon. Generally, in one embodiment, polysilicon layer 89comprises the same type of material used in gates 90, 92, 94.

FIG. 22 illustrates a cross-section of multi-state non-volatile memorycell 80 after a conventional CMP process is used to remove a portion ofpolysilicon layer 89. In one embodiment, polysilicon layer 89 and chargestorage layer 88 are removed to the tops of gates 90, 92, and 94 to formgates 91 and 93. As can be seen in FIG. 22, gates 90, 91, 92, 93, and 94are isolated from each other by only the insulating layers that form aportion of charge storage layers 86 and 88.

FIG. 23 illustrates a cross-section of multi-state non-volatile memorycell 80 after N+ source and drain regions 100 and 102 are implanted insubstrate 82. An upper portion of polysilicon gates 90-94 is removedusing an etch selective to oxide and nitride, thereby leaving a portionof charge storage layer 88 exposed. Nitride sidewall spacers 98 areformed on the sides of the exposed portions of charge storage layer 88and on exposed edges of insulating layer 97 over gates 90 and 94.Nitride sidewall spacers are formed by first depositing a thin nitridelayer. The thin nitride layer is etched to form spacers 98.

FIG. 24 illustrates a cross-section of multi-state non-volatile memorycell 80 after gates 90-94, source region 100, and drain region 102 aresilicided with metal silicide 104. Source/drain region 100 is silicidedwith silicide 105 and source/drain region 102 is silicided with silicide106. A silicide block layer (not shown) is first deposited on both sidesof the regions to be silicided in a direction perpendicular to the planeof FIG. 24 so that the silicided regions are not shorted together.Spacers 98 prevent silicide bridging on the tops of gates 90-94.

FIG. 25 illustrates a cross-section of multi-state non-volatile memorycell 80 after additional polishing to remove spacers 98 and the exposedcharge storage layer 88. In another embodiment, the additional polishingstep may not be performed. Normal processing is then performed tocomplete other features necessary to complete the device, such as forexample, contacts and additional metal layers.

A multi-state non-volatile memory cell constructed in accordance withthe above described embodiments provides significantly greater storagefor a small amount of surface area and with good data retention.

The terms “front,” “back,” “top,” “bottom,” “over,” “under” and the likein the description and in the claims, if any, are used for descriptivepurposes and not necessarily for describing permanent relativepositions. It is understood that the terms so used are interchangeableunder appropriate circumstances such that the embodiments of theinvention described herein are, for example, capable of operation inother orientations than those illustrated or otherwise described herein.

Although the invention is described herein with reference to specificembodiments, various modifications and changes can be made withoutdeparting from the scope of the present invention as set forth in theclaims below. Accordingly, the specification and figures are to beregarded in an illustrative rather than a restrictive sense, and allsuch modifications are intended to be included within the scope of thepresent invention. Any benefits, advantages, or solutions to problemsthat are described herein with regard to specific embodiments are notintended to be construed as a critical, required, or essential featureor element of any or all the claims.

The term “coupled,” as used herein, is not intended to be limited to adirect coupling or a mechanical coupling.

Furthermore, the terms “a” or “an,” as used herein, are defined as oneor more than one. Also, the use of introductory phrases such as “atleast one” and “one or more” in the claims should not be construed toimply that the introduction of another claim element by the indefinitearticles “a” or “an” limits any particular claim containing suchintroduced claim element to inventions containing only one such element,even when the same claim includes the introductory phrases “one or more”or “at least one” and indefinite articles such as “a” or “an.” The sameholds true for the use of definite articles.

Unless stated otherwise, terms such as “first” and “second” are used toarbitrarily distinguish between the elements such terms describe. Thus,these terms are not necessarily intended to indicate temporal or otherprioritization of such elements.

What is claimed is:
 1. A method of making a semiconductor device,comprising: forming an isolation region surrounding an active region,wherein the active region is in a substrate; depositing a first chargestorage layer on a top surface of the active region and over theisolation region; depositing a first layer comprising one of a groupconsisting of metal and polysilicon on the active region; etching thefirst layer to leave a first line and a second line of the first layeracross the active region and the isolation region; after etching thefirst layer, depositing a second charge storage layer over the activeregion and the isolation region; depositing a second layer comprisingone of a group consisting of metal and polysilicon over the secondcharge storage layer; removing portions of the second layer and thesecond charge storage layer to leave a third line between the first lineand the second line and separated from the first line and the secondline by a remaining portion of the second charge storage layer; forminga drain on a first side of the first line, wherein the first side of thefirst line is away from the third line; forming a source on a first sideof the second line, wherein the first side of the second line is awayfrom the third line; and doping the first line, the second line, and thethird line over the isolation region to a first conductivity type anddoping the first line, the second line, and the third line over theactive region to a second conductivity type.
 2. The method of claim 1,wherein: the depositing the first layer is further characterized asdepositing the first layer on the isolation region; the etching thefirst layer leaves the first line and the second line on the isolationregion; the depositing the second charge storage layer deposits thesecond charge storage layer over the isolation region; and the removingleaves the third line between the first and second lines and separatedfrom the first and second lines by the second charge storage layer overthe isolation region.
 3. The method of claim 2, wherein the depositingthe first layer comprises depositing polysilicon and the depositing thesecond layer comprises depositing polysilicon.
 4. The method of claim 1,wherein: the etching the first layer leaves a fourth line of the firstlayer across the active region; and the removing portions of the secondlayer and the second charge storage layer leaves a fifth line betweenthe second line and the fourth line separated from the second line andthe fourth line by a second remaining portion of the second chargestorage layer, wherein the removing portions comprises chemicalmechanical polishing.
 5. A method of making a semiconductor device,comprising: forming an isolation region surrounding an active region,wherein the active region is in a substrate; depositing a first chargestorage layer on a top surface of the active region and on the isolationregion; depositing a first layer of polysilicon on the active region;etching the first layer to leave a first line and a second line of thefirst layer across the active region and on the isolation region; afteretching the first layer, depositing a second charge storage layer overthe active region and on the isolation region; depositing a second layerof polysilicon over the second charge storage layer; removing portionsof the second layer and the second charge storage layer to leave a thirdline between the first line and the second line, wherein the third lineremains across the active region and on the isolation region, and thethird line is separated from the first line and the second line by aremaining portion of the second charge storage layer; forming a drain ona first side of the first line, wherein the first side of the first lineis away from the third line; forming a source on a first side of thesecond line, wherein the first side of the second line is away from thethird line; and doping the first line, the second line, and the thirdline over the isolation region to p type and doping the first line, thesecond line, and the third line over the active region to n type.
 6. Themethod of claim 5, wherein: the forming the isolation region furthercomprises surrounding a second active region; the depositing the firstcharge storage layer further comprises depositing the first chargestorage layer over the second active region; the etching the first layerleaves the first line and the second line on the second active region;the depositing the second charge storage layer deposits the secondcharge storage layer over the second active region; and the removingleaves the third line between the first and second lines and separatedfrom the first and second lines by the second charge storage layer overthe second active region.
 7. The method of claim 6, the doping furthercomprises doping the first line, the second line, and the third lineover the second active region to n type.
 8. A method of forming asemiconductor device, comprising: forming an isolation region around anactive region of a substrate; depositing a first charge storage layercomprising nanocrystals over the active region and the isolation region;forming a first conductive line of polysilicon and a second conductiveline of polysilicon across the active region and the isolation regionand removing the first charge storage layer between the first and secondconductive lines, wherein the first conductive line has a first sidewallfacing a first sidewall of the second conductive line; after forming thefirst conductive line and the second conductive line, depositing asecond charge storage layer comprising nanocrystals over the activeregion and the isolation region; forming a third conductive line ofpolysilicon over the second charge storage layer, the active region, andthe isolation region, the third conductive line having a pair ofsidewalls, wherein the third conductive line has a bottom surface on thesecond charge storage layer, the third conductive line is between thefirst and second conductive lines, and the second storage layer adjoinsthe pair of sidewalls of the third conductive line, the first sidewallof the first conductive line, and the first sidewall of the secondconductive line; and doping the first conductive line, the secondconductive line, and the third conductive line over the isolation regionto a first conductivity type and doping the first conductive line, thesecond conductive line, and the third conductive line over the activeregion to a different, second conductivity type.
 9. The method of claim8, wherein: the forming the isolation region further comprisessurrounding a second active region; the depositing the first chargestorage layer further comprises depositing the first charge storagelayer over the second active region; the forming the first conductiveline and the second conductive line comprises forming the firstconductive line and the second conductive line over the second activeregion; the depositing the second charge storage layer deposits thesecond charge storage layer over the second active region; and theforming the third conductive line comprises forming the third conductiveline on the second charge storage layer over the second active region.10. The method of claim 9, the doping further comprises doping the firstconductive line, the second conductive line, and the third conductiveline over the second active region to the second conductivity type.